In some conventional solar cells, impurities such as phosphor or arsenic are doped into mono or polycrystalline silicon substrates to form pn junctions. In such a solar cell, it is generally known that electrons and holes formed at the pn junction are recombined to reduce the conversion efficiency (power generation efficiency). This has led to the proposition of a selective emitter structure in which an emitter layer is doped with impurities with parts of the emitter layer to be in contact with the surface electrodes set to be doped with impurities at a higher concentration than the other parts of the emitter layer, and thereby the other parts not to be in contact with the surface electrodes locally have higher resistance.
In such a selective emitter structure, impurities are conventionally doped by a so-called coating diffusion process (a spray process) (see Patent Document 1, for example). At the coating diffusion process, a coating liquid in which n-type impurities are dissolved in an organic solvent is prepared first and is applied to the surface of a p-type substrate by using a spin coater or the like. The substrate is then subjected to annealing for thermal diffusion of the n-type impurities of the coating liquid. By the annealing, an n+ layer of high concentration is formed in the surface of the substrate, and an n layer of low concentration is formed at the position deeper than the n+ layer. Next, the region of the surface of the substrate where the surface electrodes are to be formed is covered with a mask, and part of the n+ layer not covered with the mask is removed by etching to expose the n layer. The mask is then removed.
Herein, in the coating diffusion process, the concentration distribution of impurities doped into the substrate and the depth of the impurities from the surface of the substrate are controlled by using the concentration of the coating liquid and annealing time. However, the thermal diffusion of impurities requires long processing time (generally, 30 minutes at an annealing temperature of 925° C.). Moreover, it is difficult to accurately control the depth of impurities from the substrate surface throughout the substrates to be processed.
Furthermore, it is necessary to improve the wettability of a coating liquid at applying the coating liquid to the substrate through spin coating or the like. This requires a process to previously remove naturally oxidized film in the substrate surface prior to the application of the coating liquid, thus resulting in an increase in the number of processes. Moreover, some of the coating liquid overflows to the side surface of the substrate at application of the coating liquid. This requires a process to perform edge cutting to cut off the edge of the substrate. Accordingly, use of the coating diffusion process to dope impurities has the problems of low mass-productivity at manufacturing solar cells due to a large number of processes and long processing time.
On the other hand, there is another proposition that impurities are doped in the selective emitter structure by using an ion injection apparatus for use in manufacturing semiconductor devices (see Patent Document 2, for example). However, the ion injection apparatus is typically configured to generate many kinds of ions by ion sources, separate necessary ions from the many kinds of ions by a mass separator, and then irradiate a substrate with the ion beam accelerated by an accelerator and thereby controlled to a predetermined injection energy. Accordingly, the ion injection apparatus includes a large number of parts and is expensive. Moreover, the ion injection apparatus scans the substrate surface with an ion beam in directions X and Y for injection of predetermined ions, which requires long processing time. The injection apparatus has therefore limitations on the improvement of the mass-productivity by increasing the throughput.